Fix generating html_dart2js.dart and html_dartium.dart files.

lib/html/scripts/systemhtml.py

 #!/usr/bin/python
 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 """This module provides shared functionality for the system to generate
 Dart:html APIs from the IDL database."""
 
 import emitter
 
@@ skipping 1205 matching lines @@
 
 class HtmlDart2JSSystem(System):
 
   def __init__(self, options):
     super(HtmlDart2JSSystem, self).__init__(options)
 
   def ImplementationGenerator(self, interface):
     return HtmlDart2JSClassGenerator(self, interface)
 
   def GenerateLibraries(self, dart_files):
+    auxiliary_dir = os.path.relpath(self._auxiliary_dir, self._output_dir)
     self._GenerateLibFile(
         'html_dart2js.darttemplate',
         os.path.join(self._output_dir, 'html_dart2js.dart'),
-        dart_files)
+        dart_files,
+        AUXILIARY_DIR=systembase.MassagePath(auxiliary_dir))
 
   def Finish(self):
     pass